The present invention relates to a process and apparatus for machining an electrode workpiece by means of intermittent and electro-erosive electrical discharges occurring between an electrode workpiece and an electrode tool, and more particularly to a method and apparatus whereby the electrodes are retracted relative to each other momentarily and repeatedly, as a function of the machining conditions at the gap between the electrodes.
It is known, in EDM technology, to periodically and repeatedly retract the electrodes, one relative to the other, to facilitate machining under adverse conditions in order to obtain a rapid flushing of the gap by the machining fluid for the purpose of improving the machining conditions. It is known that rapid oscillation of the electrodes is an efficient and simple arrangement for renewing the dielectric fluid in the machining gap. However, no one has been able, until now, to achieve an optimum compromise between the machining time lost during retraction of the electrodes and the shortening of machining time realized by improving the machining rate as a result of forcing the dielectric fluid to flow through the machining zone during retraction of the electrode.
Adaptive controls of the frequency and duration of the electrode intermittent retractions have already been proposed. For example, in the method described in U.S. Pat. No. 3,875,374, the amplitude and the frequency of the electrode retraction are periodically varied as a function of the electrical discharge machining parameters. Such a method, which maintains predetermined machining conditions, does not permit to achieve a maximum machining removal efficiency during each relative retraction of the electrodes.
The present invention results from the discovery that a maximum material removal rate may be achieved by taking into consideration the progressive deterioration of the machining conditions under which the electrical discharge machining is effected in the course of each machining cycle during a single relative retraction of the electrode. This progressive deterioration causes a decrease of the useful machining current, and there is a well-defined ratio between the duration of an electrode retraction and the time interval between two consecutive retractions during which the average useful current, calculated during a single retraction period and, consequently, the average machining rate during the same time interval, is maximum.
The EDM process of the present invention has for its principal object to effectuate the relative retractions of the electrodes at a permanently optimized frequency and duration of the relative retractions. The process of the invention consists principally in measuring a characteristic value of the machining rate, calculating the average value of this parameter during at least one time interval comprising a single retraction period of the electrodes, and varying the frequency of the retractions as a function of that average value, for the purpose of maintaining the machining rate at a maximum value.
The invention relates also to an apparatus for practicing the method of the invention. The apparatus comprises means for retracting the electrodes one relative to the other during a predetermined time interval, means for developing a first electrical magnitude representative of the machining rate as soon as a machining cycle is begun, means for developing a second electrical magnitude representative of the time elapsed from the instant the electrodes are retracted, means for calculating the ratio of the first electrical magnitude to the second electrical magnitude, means for detecting the instant at which this ratio ceases increasing, and means cooperating with the detection means for controlling the next retraction of the electrodes.